The mechanism of directionally selective units in rabbit's retina.

Directionally selective single units have recently been found in the cerebral cortex of cats (Hubel, 1959; Hubel & Wiesel, 1959, 1962), the optic tectum of frogs and pigeons (Lettvin, Maturana, McCulloch & Pitts, 1959; Maturana & Frenk, 1963), and the retinae of rabbits (Barlow & Hill, 1963; Barlow, Hill & Levick, 1964). The term 'directionally selective' means that a unit gives a vigorous discharge of impulses when a stimulus object is moved through its receptive field in one direction (called the preferred direction), whereas motion in the reverse direction (called null) evokes little or no response. The preferred direction differs in different units, and the activity of a set of such units signals the direction of movement of objects in the visual field. In the rabbit the preferred and null directions cannot be predicted from a map of the receptive field showing the regions yielding on or off-responses to stationary spots. Furthermore, the preferred direction is unchanged by changing the stimulus; in particular, reversing the contrast of a spot or a black-white border does not reverse the preferred direction. Hubel & Wiesel (1962) thought that the directional selectivity of the cat's cortical neurons could be explained by the asymmetrical arrangement of on and off zones in the receptive field, and the simple interaction of effects summated over these zones, but the foregoing results rule out this explanation, at least in the rabbit's retina (Barlow & Hill, 1963). In the present paper we go on from this point to describe experiments which show, first, that directional selectivity is not due to optical aberrations of some kind and, secondly, that it is not a simple matter of the latency of response varying systematically across the receptive field. After these negative results we describe experiments upon the organization of directional selectivity within the receptive field, and upon its mechanism. These lead us to the conclusion that the ganglion cells responding to a